Excessive Nitrogen Fertilization Research Articles

Modulations in carbon and nitrogen assimilation patterns in rice plants exposed to elevated atmospheric carbon dioxide concentrations

Aim: To study the influence of elevated atmospheric CO2 concentrations on the carbon and nitrogen assimilation patterns in rice plants. Methodology: Rice (Oryza sativa) plants were placed in Open Top Chambers (OTCs) and exposed to elevated levels of CO2. The treatments consisted of three levels of CO2 (398, 550 and 750 µmol mol-1) and three levels of nitrogen (0, 150 and 200 kg ha-1) and replicated five times in completely randomized design. Results: Leaf nitrogen was significantly reduced by 10.6 % and 6.5 % during later stages in rice plants exposed to CO2 @ 750 µmol mol-1 and 550 µmol mol-1, respectively over the ambient CO2. Rice plants under elevated CO2 did not exhibit any variations in Nitrate Reductase activity in leaves in comparison to ambient CO2 at tillering stage. Interestingly, NRase activity in leaves decreased at flowering stage whereas NRase activity in roots increased at same stage. The highest mean nitrogen values (0.58, 0.89 and 1.35 %) were observed in Camb (ambient CO2 concentration) and the lowest values (0.51, 0.80 and 1.27 %) in C750 in roots, straw and grains, respectively. Elevated CO2 @ 750 µmol mol-1 significantly increased the above ground biomass (straw and grain) by 15.6 and 40.1 %, respectively, over the ambient CO2 of 398 µmol mol-1. Interpretation: Elevated CO2 enhanced the grain productivity but affected the quality of rice grains. Thus, excessive nitrogen fertilization above the current recommendation is necessary for future high CO2 environments.

Growth Performance Can Be Increased Under High Nitrate and High Salt Stress Through Enhanced Nitrate Reductase Activity in Arabidopsis Anthocyanin Over-Producing Mutant Plants

Nitrogen is one of the most important macro-nutrients for plant growth and crop productivity. The amount of synthetic nitrogen fertilizers supplied to crops has dramatically increased, leading to a notable rise in crop yields. However, excessive nitrogen use has an enormous negative impact on ecosystems and human health through the emission of intense greenhouse gases, such as nitric oxide derived from the nitrate (NO3–) assimilation cascade. Additionally, owing to the development of extensive irrigation in agriculture, crops are known to suffer from high salt stress. The effect of excessive nitrogen fertilizer application has been studied in some crops, but the effect of high nitrate level and salt stress on plant stress tolerance has not been studied in detail. Therefore, in this study we aimed to study the effects of high concentrations of NO3– on salt stress tolerance in Arabidopsis. In addition, since anthocyanin functions as a reactive oxygen species (ROS) scavenger under abiotic stress conditions, we investigated whether enhanced anthocyanin content helps Arabidopsis to withstand higher salt stress levels under high NO3– concentrations by using pap1-D/fls1ko double mutant plants, which accumulate excessive amount of anthocyanin. We found that Col-0 plants are more sensitive to salt stress under high NO3– concentrations. Although both the pap1-D/fls1ko and fls1ko plants accumulated higher anthocyanin levels and radical scavenging activities than Col-0 plants under both normal and salt stress conditions, the fls1ko plants exhibited much better growth than the pap1-D/fls1ko plants. It appears that the enhanced NR activities and transcript levels of NIA1 and NIA2 in pap1-D/fls1ko and fls1ko plants led to an increase in the synthesis of proteins and proline, which increases osmolytes against salt stress. Our results demonstrate that optimal levels of anthocyanin accumulation can enhance growth performance of plants under high NO3– and salt stress conditions.

Long-term excess nitrogen fertilizer increases sensitivity of soil microbial community to seasonal change revealed by ecological network and metagenome analyses

Nitrogen (N) fertilizer has often been generously applied to increase crop biomass yield. Although the influences of inorganic N fertilizer on soil microbial communities have been widely studied, the effect of N fertilizer on microbial co-occurrence networks and its metagenome is largely unknown. Further, seasonal changes in microbial community responses to N addition have rarely been reported. In this study, three N fertilizer rates (0, 56, 196 kg N/ha) were applied annually in switchgrass (Panicum virgatum L.) grown for bioenergy production in the upper Midwest, USA. The soil microbiome was affected by both fertilizer and low pH in the 7th year of fertilization treatments. The microbial community structures were relatively stable during the growing season for each N fertilizer rate. However, the excess N fertilizer (196N) increased the seasonal variation of bacterial and fungal communities. Network analysis showed that the microbial interactions at the 196N treatment were more intense, with decreased bacteria-fungal interactions compared to 56N and 0N. This suggests that the microbial community became more sensitive to environmental change under the influence of long-term excess N fertilizer. Metagenomic analysis showed that the long-term excess N fertilizer promoted many metabolic processes, especially carbohydrate and amino acid related metabolism and Archaea mediated ammonia oxidation. However, N fertilizer also reduced many other traits, especially N2 fixation and signal transduction, the latter of which may contribute to the decreased interactions between bacteria and fungi.

Impact of Rhizobium biofertilizer on agronomical performance of lentil (BARI Masur-6) in Bangladesh

Excessive nitrogen fertilizer uses in crop field causes surface water pollution, which has a harmful effect on the ecosystem. The study was conducted to reducing the use of nitrogen fertilizers during cultivation of pulse crop and as an alternative to increase the use of biofertilizers. Rhizobium can fix atmospheric nitrogen to the soil; it can be used as an alternative to urea for the cultivation of lentil (BARI Masur-6). The Rhizobium leguminosarum was isolated from root nodules of lentil (Lens culinaris) plants and cultured in YEMA (Yeast Extract Mannitol Agar) media. The Rhizobium was screened on the ground of physiological, biochemical and environmental conditions. Different doses of urea fertilizers (20, 40, 60, 80 kg ha-1) and liquid Rhizobium were used in experimental plots. The results indicated that biofertilizer with different chemical fertilizer performed higher than application of several level of urea nitrogen fertilizer in respect of plant height, number, chlorophyll content (µg cm-1) and number of nodules plant-1 with variety BARI Masur-6. There was optimum relative growth rate (RGR) also observed. The increase in urea nitrogen levels was the reason for the decline in relative plant growth. Yield and yield contributing characters like number of pod plant-1, number of seed plant-1, 1000-grain weight (g), grain yield (t ha-1), straw yield and biological yield were significantly influenced by biofertilizer application. The assembled application of biofertilizer and chemical fertilizer produced maximum number of harvest index (%) compare to the chemical fertilizers. Significant correlation found with no of nodule, no of seed, seed weight, grain yield and straw yield. Significant correlation also found in chlorophyll content with some yield contributing characters like seed number, seed weight, grain yield and straw biomass. Further significant correlation observed between pod number and seed number, between seed weight and grain yield. Rhizobium can fulfill the alternate source of nitrogen that promoted significant growth and yield of lentil and it was much closer to the farmer’s conventional amount of urea.

Soil carbon sequestration efficiency under continuous paddy rice cultivation and excessive nitrogen fertilization in South China

Long-term excessive applications of chemical fertilizers may result in adverse impacts on soil functions. This study was to evaluate soil organic carbon (SOC) sequestration efficiency under continuous paddy rice cultivation with an excessive nitrogen (N) fertilization over the period from1980 to 2017 in South China. The SOC and total nitrogen (TN) of total 108 soil samples from continuous paddy soils and new paddy soils collected in 2017 were accordingly compared with those of 54 samples from paddy soils and upland soils obtained in 1980. Results show a total SOC increase of 0.79 g kg−1 from the initial content of 12.82 g kg−1 over a 37-year period despite an increased input of about 550 kg N ha−1yr-1 from fertilizers and 2000 kg C kg−1 yr−1 from all straw incorporation after 1990. This small C sequestration rate (or 0.021 g C kg−1 yr−1) was also observed to couple with a significantly elevated C:N ratio and badly weakened SOC-N correlation as of 2017, which is almost impossible to be revealed by normal fertilization experiments. The SOC sequestration rate of 0.145 g C kg−1 yr−1 of the new paddy soils that were developed from uplands since 1980 implies a declining tendency of SOC sequestration efficiency with rice cultivation time, which could be mainly attributed to both low soil N content and microbial activity rather than to SOC saturation. This case reminds of a need for more dedicated plot studies coupled with field observations on farmers’ routine fertilization practices to elucidate why the low soil N content remains with an excessive N fertilizer input and how N interacts with SOC in the continuous paddy soils.

Stem rot management by nitrogen and potassium fertilization and effect on grain yield and quality of rice in Uruguay

Stem rot (Nakataea oryzae) limits rice (Oryza sativa) production in Uruguay when there are high inoculum levels in the soil. This disease is particularly damaging in potassium (K)-deficient soils that receive excessive nitrogen (N) fertilization. Nevertheless, no data exist for the response to stem rot in modern rice cultivars with respect to the interaction between balanced N and K fertilization. The aim of this work was to evaluate the combined effect of K and N fertilization and a foliar phosphite (Phi) in reducing stem rot severity in rice to prevent significant yield losses. A 3-year field study was conducted in a rice-producing region of Uruguay to investigate the effects of: (1) K fertilization rate, 0 or based on the Mg:K ratio; (2) N fertilization rate, 0 or 140 kg ha–1 N; and (3) Phi treatment, with or without foliar Phi (1.775 kg ha–1 of K Phi, 71%), at growth stage R2-R3. While the experiment was conducted in a soil with sufficient potassium, K fertilization reduced the severity and percentage of rice tillers that were severely affected by stem rot, resulting in a mean yield increment of 2% to 18% depending on the year (mean 7.5%). Nitrogen fertilization increased stem rot severity in only 1 year, but resulted in a mean yield increment of 10% over the 3 years of the experiment. Foliar Phi applied at the late boot to panicle exertion stage did not reduce disease incidence or severity and failed to increase the total rice grain and milling yield.

Effects of different fertilizer applications on nitrogen leaching losses and the response in soil microbial community structure

Excessive nitrogen (N) fertilizer is often applied to soils to improve crop productivity, which leads to a reduction in N efficiency and an increase in N loss to groundwater. To investigate the effects of fertilizers on N loss, the different N forms in leachates and soil profiles were determined by leaching column experiments. The treatments included fertilizer applications of urea (U), amino acid (A), potassium nitrate (K), and manure (M), as well as a control without fertilizer (CK). The results showed that the total N loss (TL) ranged from 58.85 kg N ha −1 a−1 to 905.34 kg N ha −1 a−1, and that the percentage of N loss (PL) to the total N was between 2.09% and 32.14%. Exogenous N fertilizer addition resulted in a higher TL and higher PL, and thus lower amounts of total dissolved N (TDN) in soils. With respect to the different forms of N, NO3−-N and dissolved organic N (DON) were the main contributors to the TL, accounting for 46.96%–98.68% and 1.12%–50.88%, respectively. Notably, urea and manure addition resulted in higher DON losses, which were 2.34-fold and 1.04-fold that of CK. Furthermore, the relative abundances of nitrification-related and organic matter degradation-related bacteria in deep soil layers were 1.13–1.64 and 1.01–1.02 times higher than those in CK. The sources of DON included the direct leaching of DON contained in the fertilizers, and the enhanced DON derived from native soil organic matter through the improvement of the related microorganisms in deep soils.

Comparison of regulatory approaches for determining application limits for nitrogen fertilizer use in Germany

This study examined the suitability of three different indicators as entry points for agricultural regulation for limiting excess nitrogen (N) fertilizer inputs in Germany: net soil surface balance, gross farm-gate balance, and fertilization planning. Data on about 6000 farms in Germany were grouped into types for comparative analysis. The design of the regulatory approaches and the reliability of constituent parameters were then examined, and proportions of affected farms and mean N reduction requirements were identified. This revealed that: (a) design and purpose of the regulatory approaches differ, but the data requirements are very similar; (b) the parameters involved differ in reliability and integrity; and (c) the limits for maximum N fertilizer input at farm level vary with approach and farm type.

Excessive Nitrogen Fertilization Favors the Colonization, Survival, and Development of Sogatella furcifera via Bottom-Up Effects.

Fertilization can trigger bottom-up effects on crop plant–insect pest interactions. The intensive use of nitrogen fertilizer has been a common practice in rice production, while the yield has long been challenged by the white-backed planthopper, Sogatella furcifera (Horváth). High nitrogen fertilization can facilitate S. furcifera infestation, however, how nitrogen fertilizer leads to high S. furcifera infestation and the nutritional interactions between rice and S. furcifera are poorly understood. Here, we evaluated the effects of various levels of nitrogen fertilizer application (0–350 kg/ha) on rice, and subsequently on S. furcifera performance. We found that higher nitrogen fertilizer application: (1) increases the preference of infestation behaviors (feeding and oviposition), (2) extends infestation time (adult lifespan), and (3) shortens generation reproduction time (nymph, pre-oviposition, and egg period), which explain the high S. furcifera infestation ratio on rice paddies under high nitrogen conditions. Moreover, high nitrogen fertilizer application increased all tested rice physical indexes (plant height, leaf area, and leaf width) and physiological indexes (chlorophyll content, water content, dry matter mass, and soluble protein content), except for leaf thickness, which was reduced. Correlation analysis indicated that the specific rice physical and/or physiological indexes were conducive to the increased infestation behavior preference, extended infestation time, and shortened generation reproduction time of S. furcifera. The results suggested that nitrogen fertilizer triggers bottom-up effects on rice and increases S. furcifera populations. The present study provides an insight into how excess nitrogen fertilization shapes rice–planthopper interactions and the consequent positive effect on S. furcifera infestation.

Use of Controlled-Release Urea to Improve Yield, Nitrogen Utilization, and Economic Return and Reduce Nitrogen Loss in Wheat-Maize Crop Rotations

Excessive nitrogen (N) fertilizer input has become a common phenomenon among most farmers in the winter wheat–summer maize rotation system of north-central China, and has resulted in low nutrient use efficiency and environmental pollution. Controlled-release urea (CRU) is proposed as a solution to excessive fertilization because CRU achieves high yields and reduces N losses. Therefore, CRU mixed with normal urea at rates based on the Nutrient Expert (NE) system was used as fertilizer in a 4-year field experiment to test the preference in crop yields, economic benefits, nitrogen use efficiencies, and N losses. The following fertilizer treatments were established: local farmers’ practices (FP); normal urea fertilizer at the rate recommended by the NE system (NE); mixed CRU and normal urea at ratios of 60:40 (CRU1) and 75:25 (CRU2) based on the NE system; and 80% of the recommended N rate of the NE, CRU1 and CRU2 treatments (80%NE, 80%CRU1 and 80%CRU2). The results showed that, compared with the NE treatment at the same application rate of N, mixed CRU and urea increased yields and net benefits while reducing N loss. The application of CRU at 60% for maize and 75% for wheat had the best overall effects. Compared with FP, the average grain yield, recovery efficiency of N fertilizer and net benefits increased by 8.5%, 10.9% and 11.3%, respectively, for maize with CRU1, and increased by 4.5%, 15.1% and 10.3%, respectively, for wheat with CRU2. Furthermore, mixed CRU and urea at the recommended N rate significantly reduced N loss from 38.5% to 40.3% but increased soil NO3−-N and NH4+-N contents at 0–30 cm, although opposite results (NO3−-N) were observed deeper in the soil (30–90 cm). In the treatments 80%CRU1 and 80%CRU2, the maize yield and overall economic benefits were equivalent to those in the FP treatment, but apparent N loss was significantly reduced. Thus, these results confirmed that the combination of the CRU and the NE system for winter wheat–summer maize in north-central China is efficient and valuable, and has the potential to improve yield, nitrogen use efficiency and net benefit with low N losses.

Responses of Root Characteristic Parameters and Plant Dry Matter Accumulation, Distribution and Transportation to Nitrogen Levels for Spring Maize in Northeast China

Improving nitrogen use efficiency is a significant scientific problem to be solved. Two maize hybrids JD27 (Jidan 27) and SD19 (Sidan 19) were selected to study the effects of nitrogen levels on root characteristic parameters and plant dry matter accumulation, distribution and transportation. We set five different nitrogen levels, which were nitrogen deficiency (000N), low nitrogen (075N), medium nitrogen (150N), high nitrogen (225N) and excessive nitrogen (300N). The results showed that the root length and root surface area of JD27 were significantly higher than those of SD19 under 075N. With the increase of nitrogen levels, the root difference among varieties gradually decreased. The root length, projection area, total surface area and total volume reached the maximum values at silking stage. The average root diameter kept stable or decreased slowly with the growth stage. The dry matter accumulation of JD27 was higher than that of SD19 at all growth stages. Increasing the amount of nitrogen fertilizer can promote the transport of dry matter to grain and improve dry matter transport efficiency after anthesis. Under the treatment of medium and high nitrogen fertilizer, maize was easy to obtain a higher yield, but excessive nitrogen fertilizer inhibited the increase of yield. This study provides theoretical and practical guidance for maize production techniques.

Metagenomics reveals taxon-specific responses of the nitrogen-cycling microbial community to long-term nitrogen fertilization

Excessive nitrogen (N) fertilization in agricultural ecosystems strongly affects microbial N-cycling processes in soil. However, a comprehensive understanding of how microorganisms involved in each N transformation process respond to long-term N input is lacking. Here, using metagenomic sequencing combined with the direct assembly of N-cycling genes, we found that long-term N fertilization elevated the abundance of the microorganisms involved in most N-transforming processes but decreased that of N-fixing assemblages. The composition of the microbial groups involved in each N-transforming process was altered by fertilization, even though the abundance of several functional genes was not significantly changed. The relative abundance of microbial genera participating in the same step of N-cycling processes correlated with different soil properties, suggesting niche separation of these N-cycling taxa. Our results also indicated that the different responses to N fertilization exhibited by the taxa within the same functional group may be important for sustaining microbial nitrogen cycling in complex and dynamic environments.

Negative impacts of excessive nitrogen fertilization on the abundance and diversity of diazotrophs in black soil under maize monocropping

Excessive nitrogen (N) fertilization (ENF) and low utilization efficiency of fertilizer-derived N in high-input, high-yield cropping systems are serious ecological and economic problems in China. To examine the effects of long-term ENF on soil diazotrophs in relation to maize growth, we performed a 5-year field trial whereby 0–300 kg N ha−1 were added to black soil under a maize monocropping system in Northeast China. At the end of the 5-year field trial, the abundance and community structure of diazotrophs in the bulk soil and rhizosphere were investigated at the maize jointing stage by real-time quantitative polymerase chain reaction (qRT-PCR) and high-throughput sequencing. The results showed that: 1) ENF (N300) limited maize growth in the early stages and did not increase grain yield compared with moderate N supplementation (N180); 2) ENF significantly reduced nifH copy number, species richness, and Shannon index values of soil diazotrophs by 53.7%–79.7%, 37.2%–47.6%, and 20.0%–31.6%, respectively, reduced the relative abundance of Burkholderia, and increased the relative abundance of Sphingobium; 3) ENF decreased soil pH and increased electrical conductivity, which were the main factors affecting soil diazotrophic community structure as determined by Mantel test; and 4) diazotrophs were 1.3–3.0 times more abundant in the rhizosphere than in bulk soil, but no differences in α-diversity and community composition of diazotrophs were observed between rhizosphere and bulk soil. Thus, long-term ENF has negative impacts on the soil diazotrophic community, inhibiting maize growth in the early stage. Split N fertilization or slow/controlled-released N fertilizer should be used to avoid the negative effects of ENF on the soil diazotrophic community structure and early maize growth.

Hayvan Yemi Olarak Kullanılan Şekerpancarı Yapraklarının Nitrat İçeriğine Azot-Bor Uygulamasının ve Zamanın Etkisi

Şekerpancarı üretiminde verim ve kalite için azot (N) ve bor (B) gübrelemesi önemlidir. Fakat fazla azotlu (N) gübreler kalite parametreleri üzerinde olumsuz etkilere neden olmaktadır. Hayvan yemi olarak kullanılan şeker pancarı yapraklarında nitrat birikiminin artması da olumsuz etkilenen kalite parametrelerinden birisidir. Bu çalışmanın amacı, şeker pancarına farklı dozda azot ve bor uygulamasının bitki yapraklarında nitrat birikimi üzerine etkisini incelemektir. Çalışma, Kahramanmaraş ili Elbistan İlçesinde 2017 yılında B içeriği 0,56 mg kg-1 olan bir arazide yürütülmüştür. Deneme tesadüf blokları faktöriyel deneme desenine göre 3 tekerrürlü olarak kurulmuştur. Denemede 5 farklı N dozu (0, 9,18, 27, 36 kg N da-1) ve 4 farklı B dozu (0, 200, 400, 600 g B da-1) uygulanmıştır. Araştırmada temmuz, ağustos, eylül ve ekim aylarında yaprak örnekleri alınmıştır. Araştırma sonuçlarına göre N dozu arttıkça yaprakların nitrat içeriği istatistiksel olarak önemli derecede artmıştır. Bor uygulamasının yaprakların nitrat içeriği üzerine etkisi önemli bulunmamıştır. Hasat dönemine doğru yaprakların nitrat içeriği istatistiksel olarak önemli düzeyde azalmıştır. Sonuç olarak hasat sonrası şekerpancarı yapraklarının hayvan yemi olarak kullanması, nitrat bakımından hayvanlar için bir risk oluşturmamaktadır.

Exponential relationship between N2O emission and fertilizer nitrogen input and mechanisms for improving fertilizer nitrogen efficiency under intensive plastic-shed vegetable production in China: A systematic analysis

Currently, China has approximately four million hectares of intensively cultivated plastic-shed vegetable production, i.e., with excessive nitrogen (N) fertilization and high irrigation. Plastic-shed vegetable production has helped meet the rapidly increasing consumer demand for fresh vegetables while improving heat, light, and land utilization efficiencies, resulting in very high vegetable yield. We collected all studies from the 1980s to 2020 on N2O emissions and N fertilization associated with plastic-shed vegetable production at 40 field sites in China. Fertilizer N utilization efficiencies and N2O emissions that were affected by fertilizer N rate and type, irrigation, growth duration, nitrification inhibitors, and soil properties were systematically examined. The results revealed that fertilizer N efficiencies in plastic-shed vegetable production significantly decreased with increasing N fertilization rate. The average N recovery efficiency (REN) and apparent N use efficiency (ANUE) were 6.8 % and 33 %, respectively; much lower than those of cereal crop production in the same region. In fruit and leafy vegetable production, N2O emissions exhibited an exponential and linear relationship with the fertilizer N rate, respectively, and the average contributions of fertilizer N to N2O emissions were 68 % and 70 %, respectively. Compared with synthetic N fertilizer or manure alone, combination of synthetic fertilizer with animal manure significantly increased the N2O emissions and emission factors (EFs) at high N fertilization rates (>800 kg N ha−1 season−1), but there were no significant differences among fertilizer types at N rate <800 kg N ha−1 season−1. Nitrification inhibitors reduced N2O emissions by 24.0 % (95 % confidence intervals [CI]: 19.2 %–28.9 %), and water-saving irrigation is the other effective measure to reduce emissions. Plastic-shed soils with neutral pH, high organic carbon content (> 30 g kg−1), growth period of >100 d, and higher irrigation increased the responses of N2O emission to N fertilization. As plastic-shed soils are continuously and intensively farmed, soil properties will be negatively affected, and should be considered together with fertilization and irrigation to maintain high vegetable yield and low N2O emissions. Our study highlighted that the exponential relationship was more appropriate to predict the N2O emissions in plastic-shed vegetable production, and our findings help to optimize fertilizer N input with consideration of crop yield and greenhouse gas emission.

Agricultural Innovation and Sustainable Development: A Case Study of Rice–Wheat Cropping Systems in South Asia

The rice–wheat cropping system is the main food bowl in Asia, feeding billions across the globe. However, the productivity and long-term sustainability of this system are threatened by stagnant crop yields and greenhouse gas emissions from flooded rice production. The negative environmental consequences of excessive nitrogen fertilizer use are further exacerbating the situation, along with the high labor and water requirements of transplanted rice. Residue burning in rice has also severe environmental concerns. Under these circumstances, many farmers in South Asia have shifted from transplanted rice to direct-seeded rice and reported water and labor savings and reduced methane emissions. There is a need for opting the precision agriculture techniques for the sustainable management of nutrients. Allelopathic crops could be useful in the rotation for weed management, the major yield-reducing factor in direct-seeded rice. Legume incorporation might be a viable option for improving soil health. As governments in South Asia have imposed a strict ban on the burning of rice residues, the use of rice-specific harvesters might be a pragmatic option to manage rice residues with yield and premium advantage. However, the soil/climatic conditions and farmer socio-economic conditions must be considered while promoting these technologies in rice-wheat system in South Asia.

Replacing nitrogen fertilizer with nitrogen-fixing cyanobacteria reduced nitrogen leaching in red soil paddy fields

Excessive chemical nitrogen (N) fertilizer application and flood irrigation in rice production can lead to substantial soil-N losses and low nitrogen-use-efficiency (NUE) in rice due to N leaching from red paddy soils. Although inoculation of irrigated fields with nitrogen-fixing cyanobacteria (NFC) has been shown to increase rice yield, there are few studies on N leaching and NUE in response to NFC when used in place of urea. A two-year field experiment was carried out to examine the concentrations of ammonium-N (NH4+-N), nitrate-N (NO3−-N), total dissolved N (TDN) and dissolved organic N (DON) in percolated irrigation water, as well as cumulative leaching losses after NFC was used to replace different application rates of urea in double cropped rice grown in red paddy soil. Four treatments were established: 1) CK, the control with no application of urea or NFC; 2) N10C0, a conventional full application of urea only; 3) N5C5, addition of a half-application of NFC to replace half of the conventional application rate of urea (N5C5); and 4) N0C10, a full application of NFC only. In 2017 and 2018, although the average rice yield of N0C10 was 25.0 % higher than that of CK, it was 23.8 % lower than that of N10C0. The average NUE of N10C0 was 34.7 %, and the average TDN leaching loss was 13.9 kg ha-1 per crop each year. Correspondingly, NUE of N5C5 was 47.7 %, which was 37.5 % higher than that of N10C0. Replacing 50 % of urea with NFC did not significantly decrease rice yield, but it reduced TDN leaching by 37.2 %. The results showed that the conventional way of fertilization caused the most N loss, while replacing partial urea with NFC significantly reduced the leaching losses of NH4+-N, NO3−-N and DON. Moreover, replacing 50 % of urea with NFC can retain more soil N than conventional fertilization at the end of the rice season because NFC can intercept, fix and delay release of N. Thus, the multiple functions of NFC in N cycling dynamics are more conducive to the retention of N by soil and greatly reduces N leaching.

Technological and institutional lock-in and excessive synthetic nitrogen fertilizer use on North American grain and oilseed farms

This intervention examines commodity grain and oilseed farmers’ over reliance on synthetic nitrogen fertilizer in North America. Most grain and oilseed farmers apply synthetic nitrogen fertilizer at rates higher than necessary in order to ensure maximum yields. At the same time, high fertilizer application rates lead to increased farm input expenses and generate significant amounts of water pollution and excessive greenhouse gas emissions. A number of low-cost alternative approaches have been developed which can significantly reduce or eliminate the need for synthetic nitrogen fertilizer while maintaining farm profitability. But such practices have only seen limited adoption by Canadian and US farmers. This is despite significant production cost savings and environmental benefits. A number technological and institutional factors work in combination to lock farmers into production models requiring large amounts of synthetic nitrogen fertilizer. They include crop varieties bred to thrive in artificially high nitrogen soil conditions, conventional tillage practices, restrictive financial arrangements, largely unenforced water quality laws, and non-diverse marketing outlets. These technological and institutional lock-ins are significant barriers to the adoption of alternative crop production practices that are less reliant upon synthetic nitrogen fertilizer.

Transcriptional regulators of nitrate metabolism: Key players in improving nitrogen use in crops

Green revolution has boosted crop yields by the development of varieties which rely on high fertilizer application. Since then, higher productivity has largely witnessed excessive nitrogen (N) fertilizer application resulting in many environmentally and agronomically unsustainable consequences. One possible solution to this problem is to develop varieties with efficient N use endowed with genetically superior N metabolizing machinery, thereby significantly reducing N loss in soil and facilitating gainful yield performance at lower N conditions. Nitrate (NO3−) is the major form of N acquired by plants in aerobic soils. Hence, its efficient acquisition, transport, assimilation into complex organic compounds, and overall homeostasis is crucial to ensure productivity under optimal and suboptimal N conditions. Transcription factors are prime regulators of these processes, and insights into their mechanism of action and the resultant effect on N metabolism are crucial to generating crops with efficient and durable nitrogen use efficiency. The present review, therefore, presents a comprehensive updated account of major N responsive transcription factor families, their cross-talk with other growth factors, and explores existing and potential areas of their biotechnological application to maximize crop yields.

Irrigation and fertilization management to optimize rice yield, water productivity and nitrogen recovery efficiency

Increasing water scarcity and environmental contamination with excess chemical nitrogen fertilizer use necessitate the development of water-nitrogen conservation technology in rice production. Therefore, a 2-year field experiment (2017–2018) was conducted with three water regimes, namely (1) continuous flooding irrigation, CF; (2) safe alternate wetting and drying irrigation, AWDsafe; and (3) severe alternate wetting and drying irrigation, AWDsevere, and four nitrogen application (Napp) rates, namely 0 (N0), 90 (N1), 180 (N2), and 270 (N3) kg N ha−1, to determine the effects of water regimes and Napp rates on rice yield, total water productivity (WPi+r) and nitrogen recovery efficiency (NRE). The results demonstrated that the water regime, Napp rate and their interaction showed significant effects on rice yield, WPi+r and NRE and similar variations were observed in 2017 and 2018. The rice grain yield and WPi+r (or the water productivity of irrigation, WPi) significantly increased from N0 to N2 treatments but varied little between N2 and N3 treatments. The rice yield under AWDsafe was higher than that under AWDsevere, whereas their WPi+r and WPi values showed the opposite trends. The WPi values in 2018 were substantially higher than those in 2017 due to the lower irrigation amount in 2018. The highest rice NRE occurred with the combination of N2 with the CF and AWDsafe conditions, and it was significantly higher than that under AWDsevere. The dualistic and quadric regression equations of water and Napp rate showed that rice yield, WPi+r and NRE could not be maximized simultaneously. Based on the maximum likelihood method, it was demonstrated that maintaining the water quantity and Napp rate at 11,000 m3 ha−1 and 160 kg N ha−1 can serve as a suitable strategy to achieve maximal comprehensive benefits for rice grain yield, WPi+r and NRE in certain regions with water shortage. The optimization model can save approximately 17.0% of water input and 11.1% of Napp rate, respectively, compared to the traditional strategy. However, further research should validate and adapt these technologies in larger-scale fields.